This invention relates to electrophotographic color printers that use raster output scanners. In particular, it relates to rapidly sensing and controlling the position of a scanning laser beam.
Electrophotographic marking is a well-known method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired final image onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to produce an electrostatic latent image of the desired image on the photoreceptor""s surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic marking machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, called the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various latent images and consequently the color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be performed in a various ways. For example, in a single pass printer wherein the composite image is produced in a single cycle of the photoreceptor. This requires a charging, an exposing, and a developing station for each color of toner. Single pass printers are advantageous in that they are relatively fast since a composite color image can be produced in one cycle of the photoreceptor.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. That beam reflects from the facets and strikes the photoreceptor, producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. The direction of the sweeping spot is called the fast scan direction. By moving the photoreceptor perpendicular to the fast scan direction, as the polygon rotates the spot raster scans the entire photoreceptor. The direction of motion of the photoreceptor is referred to either as the slow scan direction or the process direction. During scanning, the intensity of the laser beam is modulated to produce the desired latent image.
In color electrophotographic printing it is very important that the various color latent images be accurately registered with each other. By registration it is meant that the latent images are produced such that when the various latent images are developed and transferred that the desired composite image results. Registration must be performed in both the process (slow scan) direction and in the fast scan direction. Misregistration causes color errors that are highly noticeable by the human eye.
Various factors lead to misregistration. For example, photoreceptor motion may not be perfect because vibration, motor backlash, gear train interactions, mechanical imbalances, and/or friction, among other factors, can cause the instantaneous position of the photoreceptor to be less than ideal. Another problem is phasing errors. Phasing errors come about because it is very difficult to accurately synchronize the rotation of the polygon with the motion of the photoreceptor. When the photoreceptor is in the proper position to receive the latent image the polygon facet that should reflect the laser beam might be misregistered xc2x1xc2xd of a scan line in the slow scan, i.e., process direction. The result is a misplacement of the image. Another source of misregistration is polygon facet variations. While each polygon facet is desired to be identical to every other facet, in practice this ideal is not met. Variations in facet dimensions, surface characteristics, and facet-to-facet angular variations cause the scan line position to be facet-dependent. Significantly, the present invention is particularly useful in addressing facet-dependent scan line position problems.
Misregistration in the fast scan direction is usually reduced using a start-of-scan sensor that detects when the sweeping spot is at a predetermined location. Using location information the modulation of the laser beam can be controlled such that the latent image starts at the correct fast scan direction location. However, misregistration in the slow scan direction is more difficult to reduce. One approach is to accurately control the photoreceptor""s motion. However, because of inertia, backlash, and other mechanical motion problems, as well as phasing errors, this is difficult and expensive to do.
Another approach to reducing slow-scan direction misregistration is xe2x80x9caerialxe2x80x9d image control. With aerial image control, instead of precisely controlling the photoreceptor and ROS motions, those elements are allowed to vary slightly and the scan line position is adjusted to reduce misregistration. For example, U.S. Pat. No. 5,287,125 to Appel et al. discloses a raster output scanner that has process direction (slow scan direction) scan line position control. In that patent an error feedback circuit senses the position of a moving photoreceptor. Photoreceptor position errors are used to produce signals that are applied to a piezoelectric actuator. The piezoelectric actuator expands or contracts, moving a pre-polygon lens, which moves the scan line produced on the photoreceptor so as to correct for photoreceptor motion errors. Additionally, U.S. patent application Ser. No. 09/004,762 now U.S. Pat. No. 6,023,286, entitled xe2x80x9cMOVING MIRROR MOTION QUALITY COMPENSATION,xe2x80x9d filed on Jan. 8, 1998 and U.S. patent application Ser. No. 09/210,188, now U.S. Pat. No. 6,141,031, filed on Dec. 11, 1998 and entitled xe2x80x9cAERIAL COLOR REGISTRATIONxe2x80x9d teach piezoelectric moved mirrors that aerially correct for photoreceptor motion errors. Also see U.S. patent application Ser. No. 09/004,455 now U.S. Pat. No. 6,055,005 entitled xe2x80x9cCOLOR PRINTER WITH JITTER SIGNATURE MATCHING.xe2x80x9d
While the references cited above are useful, they have their limitations. In particular they do not correct for facet-dependent position errors. However, the scan line adjustment technique taught in U.S. patent application Ser. No. 09/210,188, which corrects for both photoreceptor position errors and for facet phasing errors, is potentially fast enough to dynamically correct for facet-to-facet variations.
However, to dynamically correct for facet-to-facet variations it is necessary to sense the laser beam""s position on the photoreceptor and to correct that scan line position such that the resulting image is properly positioned before the image is actually produced. In practice, the time available to sense and correct is very small.
A prior art scan line sensor is taught in U.S. Pat. No. 5,386,123, by inventors Hubble III et al., issued Jan. 31, 1995, and entitled, xe2x80x9cBeam Steering Sensor for a Raster Scanner Using a Lateral Effect Detecting Device.xe2x80x9d While the teachings of U.S. Pat. No. 5,386,123 are beneficial, those teachings result in sensing the average position of all scan lines over a relatively long (say 20 millisecond) time span. Therefore, the teachings of U.S. Pat. No. 5,386,123 are not suitable for sensing the scan line position from each facet.
Therefore, a new sensor capable of sensing the scan line position from each facet of a multi-faceted raster output scanner polygon would be beneficial. Even more beneficial would be a closed loop control system that senses and corrects the scan line position for each facet of a multi-faceted polygon.
The principles of the present invention provide for fast scan line position sensing systems, for closed loop scan line position control systems that incorporate fast scan line position sensing systems, and for electrophotographic printers having multifaceted polygon raster output scanners and closed loop scan line position control systems that incorporate fast scan line position sensing systems. Electrophotographic printers according to the principles of the present invention beneficially use their closed loop scan line position control systems to correct the scan line positions for each facet of their multifaceted polygon.
A fast scan line position sensing system according to the principles of the present invention includes a multi-electrode, lateral effect, photodiode sensor that senses the position of scan lines in real time. The fast scan line position sensing system further includes a signal processing circuit. The signal processing circuit includes amplifiers that amplify the photodiode signals, a highpass filter for each amplifier, and an integrator for each highpass filter. The integrators sum the outputs of the highpass filters for each individual scan associated with a single facet. High-speed sample-and-hold circuits temporarily store the integrator outputs. The sum and difference of the signals on the sample-and-hold circuits are then determined by a summing circuit and by a difference circuit. A ratio circuit then determines the ratio of the difference to the sum. The result is a position signal usable for rapidly correcting the scan line position. The signal processing circuit further includes a logic circuit for producing digital signals that control various required timing functions, such as resetting the integrators, enabling the sample-and-hold circuits, and providing a data valid indication.
A closed loop scan line control system according to the principles of the present invention includes a raster output scanner that has a laser for producing a laser beam, and a multifaceted polygon for sweeping that laser beam along a scan line. The closed loop scan line control system further includes a lateral effect photodiode sensor that senses the scan line position and a signal processing circuit for rapidly producing a position signal that depends upon the position of the scan line. The closed loop scan line control system further includes a scan line correction mechanism that adjusts the position of the scan line such that the position of the scan line is corrected for each facet of the polygon. Beneficially, the scan line correction mechanism uses a mover that moves an optical element that adjusts the scan line position.
An electrophotographic printer according to the principles of the present invention includes a moving photoreceptor and a laser-based, raster output scanner having a multifaceted polygon that sweeps the laser beam in a scan line across the photoreceptor. The electrophotographic printer further includes a lateral effect photodiode sensor that senses the position of the scan line, a signal processing circuit for processing the information from the photodiode sensor, and a scan line correction mechanism that corrects the position of the scan line.